The invention relates generally to optical transmission systems, and more particularly to a transmitter for transmitting wavelength division multiplexed optical signals through an optical communication system.
Signals on optical fiber transmission lines characterized by large bit rate distance products, such as undersea or transcontinental terrestrial lightwave transmission systems and which employ optical amplifiers are subject to a host of impairments that accumulate along its length. The source of these impairments within a single data channel include amplified spontaneous emission (ASE) noise generated in the erbium-doped fiber amplifiers (EDFAs), polarization dependent gain caused by hole burning in the EDFAs, polarization dependent loss (PDL) in the passive components, nonlinear effects resulting from the dependence of the refractive index of single-mode fiber on the intensity of the light propagating therethrough, and chromatic dispersion which causes different optical frequencies to travel at different group velocities. In addition, for wavelength division multiplexed (WDM) systems in which a plurality of optical channels are transmitted on the same optical fiber, crosstalk between channels caused by the fiber""s nonlinear index and incomplete channel selection at the receiving terminal must be considered.
WDM systems often employ transmitters that impart phase modulation to the carrier wavelengths. The phase modulation increases the spectral width of each channel so that its peak intensity is reduced, thus reducing the adverse effects of nonlinear interactions. Such transmitters may also include a polarization scrambler for varying the polarization states of the carrier wavelengths. As discussed in more detail in U.S. Pat. No. 5,526,162, for example, polarization scrambling is advantageous because it minimizes the effects of polarization hole burning. Polarization scrambling is accomplished by splitting the signal into two equal orthogonal components, applying phase modulation to one of the components and then recombining the components.
Unfortunately, the phase modulation imparted to the carrier wavelengths to reduce nonlinear interactions and polarization hole burning limits the number of channels that fit within a given bandwidth because phase modulation increases the spectral width of each channel. Of course, to increase transmission capacity, it is desirable to use as many channels as possible over the limited bandwidth that is available, thus requiring that the spectral width of the individual channels be kept to a minimum.
Accordingly, it would be desirable to impart sufficient phase modulation to the optical carrier wavelengths in a WDM transmission system to substantially reduce nonlinear interactions and polarization hole burning while avoiding any excess phase modulation so that the spacing between individual channels is reduced to increase system capacity.
In accordance with the present invention, an apparatus is provided for transmitting an optical signal through an optical fiber. The apparatus includes an optical signal source, which generates an optical signal having a plurality of optical channels onto which data is modulated. Each of the optical channels is defined by a different carrier wavelength. A phase modulator imparts phase modulation to the plurality of optical channels so that channels nearest a zero dispersion wavelength of the optical fiber are more closely spaced to one another than channels farthest in wavelength from the zero dispersion wavelength of the optical fiber.
The optical channels closest to the zero dispersion wavelength exhibit relatively low nonlinearities and hence do not require as a great a reduction in intensity as channels more remote from the zero dispersion wavelength. That is, these channels require less spectral broadening by phase modulation to reduce their peak intensities. Since those optical channels closest to the zero dispersion wavelength require the least amount of phase modulation, the present invention advantageously reduces any excess phase modulation so that the individual channels may be more closely situated to one another, increasing transmission capacity within the operating bandwidth.
In accordance with another aspect of the invention a method is provided for transmitting an optical signal. In particular, an optical signal is generated which has a plurality of optical channels onto which data is modulated. Each of the optical channels is defined by a different carrier wavelength. Phase modulation is imparted to each of the plurality of optical channels based on its spectral distance from a zero dispersion wavelength of the optical fiber. In one particular embodiment of the invention, the phase modulation is arranged so that increasing phase modulation is imparted to optical channels increasingly remote in wavelength from the zero dispersion wavelength of the optical fiber.